The GPU: Apple's Gift to Game Developers

The GPU side of the A5 is really what's most exciting. As we mentioned in our iPad 2 GPU Performance analysis, the A5 includes a dual-core PowerVR SGX 543 - also known as the SGX 543MP2. In our earlier article we showed the SGX 543MP2 easily beating both an iPad 1 and the Tegra 2 based Motorola Xoom.

To understand why the SGX 543MP2 has such a performance advantage we need to first remember that NVIDIA's Tegra 2 is nearly a year late. NVIDIA's first competitive ultra mobile GPU was supposed to be shipping in products in the first half of 2010, instead it found itself shipping in 2011. While NVIDIA is good at designing GPUs, it's not good enough that it can release a product and maintain a two year performance advantage over the competition. Let's look at the architecture, shall we?

NVIDIA's Tegra 2 features a DirectX 9-class GPU. NVIDIA used to call it the GeForce ULP (Ultra Low Power) but now it's just GeForce. As a DX9 class GPU we're dealing with a conventional, non-unified shader architecture. While all OpenGL ES 2.0 GPUs can execute pixel and vertex shader instructions, the GeForce in Tegra 2 runs pixel and vertex shaders on separate groups of hardware.

NVIDIA calls each pixel and vertex shader ALU a core. The Tegra 2 has four pixel shader cores and four vertex shader cores. The four pixel shader ALUs make up a single Vec4 and the same goes for the four vertex shader ALUs. NVIDIA wouldn't elaborate on what limitations exist when dispatching operations to the cores. All pixel shader operations happen at 20-bits per component precision while all vertex shader operations happen at 32-bits per component.

Each core is capable of executing one multiply+add (MAD) operation per clock. Do the math and that works out to be a peak rate of 8 MADs per clock for the entire GPU. The maximum operating frequency for the Tegra 2 GeForce GPU is 300MHz, however device vendors may run the GPU at a lower frequency to save on power. At 300MHz this works out to be 4.8 GFLOPS (counting a MAD as two FLOPs).

Imagination Technologies' PowerVR SGX 543MP2 is fundamentally a bigger GPU than the GeForce in NVIDIA's Tegra 2. Let's go through the math.

The SGX 543 features four USSE2 pipes. This is a unified shader architecture so both vertex and pixel shader code runs on the same set of hardware. The benefit of this approach is you get better performance in peaky situations where you're running a lot of vertex or pixel shader code and not a balance that's perfectly tailored to your architecture. The Tegra 2 will only run at peak efficiency if it encounters a mix of 50% vertex and 50% pixel shader code. The PowerVR SGX series will never have any of its execution pipes idle regardless of the instruction mix.

Each USSE2 pipe has a 4-wide vector ALU capable of cranking out 4 MADs per clock. Two of these pipes is enough to equal the peak throughput of what NVIDIA built in Tegra 2, but the PowerVR SGX 543 has four of them. As for the MP2? Go ahead and double that number again. The SGX 543MP2 is simply two 543s placed next to one another.

All of this works out to be 16 MADs per clock for the SGX 543 and 32 MADs per clock for the SGX 543MP2. At 200MHz that's 12.8GFLOPS and at 250MHz we're talking about 16 GFLOPS.

Mobile SoC GPU Comparison
  PowerVR SGX 530 PowerVR SGX 535 PowerVR SGX 540 PowerVR SGX 543 PowerVR SGX 543MP2 GeForce ULP Kal-El GeForce
SIMD Name USSE USSE USSE USSE2 USSE2 Core Core
# of SIMDs 2 2 4 4 8 8 12
MADs per SIMD 2 2 2 4 4 1 ?
Total MADs 4 4 8 16 32 8 ?
GFLOPS @ 200MHz 1.6 GFLOPS 1.6 GFLOPS 3.2 GFLOPS 6.4 GFLOPS 12.8 GFLOPS 3.2 GFLOPS ?
GFLOPS @ 300MHz 2.4 GFLOPS 2.4 GFLOPS 4.8 GFLOPS 9.6 GFLOPS 19.2 GFLOPS 4.8 GFLOPS ?

At its lowest expected clock speed, the 543MP2 already has over twice the compute power of the Tegra 2's GPU at its highest operating frequency. Take into account the fact that the A5 likely has more memory bandwidth than Tegra 2 and the SGX 543MP2 is a tile based architecture with lower bandwidth requirements and the performance numbers we talked about last time shouldn't be all that surprising.

The real competition for the SGX 543MP2 will be NVIDIA's Kal-El. That part is expected to ship on time and will feature a boost in core count: from 8 to 12. The ratio of pixel to vertex shader cores is not known at this point but I'm guessing it won't be balanced anymore. NVIDIA is promising 3x the GPU performance out of Kal-El so I suspect that we'll see an increase in throughput per core.

GPU Performance

Taken from our iPad 2 GPU Performance Preview:

As always we turn to GLBenchmark 2.0, a benchmark crafted by a bunch of developers who either have or had experience doing development work for some of the big dev houses in the industry. We'll start with some of the synthetics.

Over the course of PC gaming evolution we noticed a significant increase in geometry complexity. We'll likely see a similar evolution with games in the ultra mobile space, and as a result this next round of ultra mobile GPUs will seriously ramp up geometry performance.

Here we look at two different geometry tests amounting to the (almost) best and worst case triangle throughput measured by GLBenchmark 2.0. First we have the best case scenario - a textured triangle:

Geometry Throughput - Textured Triangle Test

The original iPad could manage 8.7 million triangles per second in this test. The iPad 2? 29 million. An increase of over 3x. Developers with existing titles on the iPad could conceivably triple geometry complexity with no impact on performance on the iPad 2.

Now for the more complex case - a fragment lit triangle test:

Geometry Throughput - Fragment Lit Triangle Test

The performance gap widens. While the PowerVR SGX 535 in the A4 could barely break 4 million triangles per second in this test, the PowerVR SGX 543MP2 in the A5 manages just under 20 million. There's just no competition here.

I mentioned an improvement in texturing performance earlier. The GLBenchmark texture fetch test puts numbers to that statement:

Fill Rate - Texture Fetch

We're talking about nearly a 5x increase in texture fetch performance. This has to be due to more than an increase in the amount of texturing hardware. An improvement in throughput? Increase in memory bandwidth? It's tough to say without knowing more at this point.

Apple iPad vs. iPad 2
  Apple iPad (PowerVR SGX 535) Apple iPad 2 (PowerVR SGX 543MP2)
Array test - uniform array access
3412.4 kVertex/s
3864.0 kVertex/s
Branching test - balanced
2002.2 kShaders/s
11412.4 kShaders/s
Branching test - fragment weighted
5784.3 kFragments/s
22402.6kFragments/s
Branching test - vertex weighted
3905.9 kVertex/s
3870.6 kVertex/s
Common test - balanced
1025.3 kShaders/s
4092.5 kShaders/s
Common test - fragment weighted
1603.7 kFragments/s
3708.2 kFragments/s
Common test - vertex weighted
1516.6 kVertex/s
3714.0 kVertex/s
Geometric test - balanced
1276.2 kShaders/s
6238.4 kShaders/s
Geometric test - fragment weighted
2000.6 kFragments/s
6382.0 kFragments/s
Geometric test - vertex weighted
1921.5 kVertex/s
3780.9 kVertex/s
Exponential test - balanced
2013.2 kShaders/s
11758.0 kShaders/s
Exponential test - fragment weighted
3632.3 kFragments/s
11151.8 kFragments/s
Exponential test - vertex weighted
3118.1 kVertex/s
3634.1 kVertex/s
Fill test - texture fetch
179116.2 kTexels/s
890077.6 kTexels/s
For loop test - balanced
1295.1 kShaders/s
3719.1 kShaders/s
For loop test - fragment weighted
1777.3 kFragments/s
6182.8 kFragments/s
For loop test - vertex weighted
1418.3 kVertex/s
3813.5 kVertex/s
Triangle test - textured
8691.5 kTriangles/s
29019.9 kTriangles/s
Triangle test - textured, fragment lit
4084.9 kTriangles/s
19695.8 kTriangles/s
Triangle test - textured, vertex lit
6912.4 kTriangles/s
20907.1 kTriangles/s
Triangle test - white
9621.7 kTriangles/s
29771.1 kTriangles/s
Trigonometric test - balanced
1292.6 kShaders/s
3249.9 kShaders/s
Trigonometric test - fragment weighted
1103.9 kFragments/s
3502.5 kFragments/s
Trigonometric test - vertex weighted
1018.8 kVertex/s
3091.7 kVertex/s
Swapbuffer Speed
600
599

Enough with the synthetics - how much of an improvement does all of this yield in the actual GLBenchmark 2.0 game tests? Oh it's big.

GLBenchmark 2.0 Egypt

Without AA, the Egypt test runs at 5.4x the frame rate of the original iPad. It's even 3.7x the speed of the Tegra 2 in the Xoom running at 1280 x 800 (granted that's an iOS vs. Android comparison as well).

GLBenchmark 2.0 Egypt - FSAA

With AA enabled the iPad 2 advantage grows to 7x. In a game with the complexity of the Egypt test the original iPad wouldn't be remotely playable while the iPad 2 could run it smoothly.

The Pro test is a little more reasonable, showing a 3 - 4x increase in performance compared to the original iPad:

GLBenchmark 2.0 PRO

GLBenchmark 2.0 PRO - FSAA

While we weren't able to reach the 9x figure claimed by Apple (I'm not sure that you'll ever see 9x running real game code), a range of 3 - 7x in GLBenchmark 2.0 is more reasonable. In practice I'd expect something less than 5x but that's nothing to complain about.

The Right SoC at the Right Time: Apple's A5 Battery Life
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  • podperson - Monday, March 21, 2011 - link

    Just admit that most PCs are used as toys. Heck, the whole reason the personal computer took off (in homes) was as a games platform.

    Most of the people I see with PCs are using them to surf the web, watch youtube, update facebook, or mess around with digital media. Where I work there are Macs and PCs available to the public with 27" monitors all open to Facebook (hint, it's a university). Exactly what is this "work" you need to do on PCs? For most people it's a little bit of text editing now.

    For some kinds of things the iPad is markedly superior ergonomically to a PC (or even a tablet computer or WACOM tablet display) — e.g. sketching or various musical apps. For others a PC is markedly superior. For still others one or the other is completely useless.
    Reply
  • michael2k - Monday, March 21, 2011 - link

    Except it isn't bulky nor underpowered for many things.

    I have a 2006 G4 iBook that is lower performance than a 2010 iPad 2. If the iPad 2 is a toy, then so is just about any early 2006 computer, including older Pentium M based laptops.

    It is also far less bulky than self same 4 year old computers, with trivially 2 to 3 times the battery life.

    I paid $500 so that my wife can follow my kids around, but still have a computer she can put in her purse. Without the iPad, she would have indeed settled for an iPod touch, but a netbook with a hinge? Too short a battery life and too hard to manage (Windows XP, Windows Update, AV, etc) for the harried housewife/homemaker
    Reply
  • bigboxes - Monday, March 21, 2011 - link

    Just how big is her purse? As for battery life I think you are looking through rose colored glasses in emphasizing the positive qualities that your device holds. As long as the device lasts until you get home to plug it in (maybe even your vehicle) it will suffice. The iPad is too bulky and not functional enough too do day to day tasks. As I said earlier, the authors point this out.

    As much as we want these cute devices to succeed we find ourselves using other devices that are far more practical. I've made the same mistake myself in the past. Anyone remember the Sony Clie? Another proprietary underpowered overpriced device. I believe I paid $500 for it. It gathered dust for years until I finally put it in a box. There's the cool factor and then there's reality. Do you set it out for your friends' visits or do you actually get x value out of it?

    Also, you are going to be carrying your phone with you already. Why carry both devices with you when one doesn't have more functionality over the other? I would think that the balance for function belongs to the smartphone (phone service is more valuable than screen size).
    Reply
  • michael2k - Monday, March 21, 2011 - link

    Her purse is big enough to hold an iPad, a wallet, another smaller purse, a phone, keys, two Capri Suns, two candy bars, a small bag of chips, and a couple of diapers.

    As for battery life, that's exactly what the iPad is; it lasts as long as it needs to until it gets home to be plugged in. I cannot find a laptop under 2 pounds with similar battery life. The minimum requirement is 6 hours.

    I carry my phone because I am more like Anand than not. She carries the iPad because she isn't like Anand, at all. It would be the equivalent of me driving a Civic and her driving a minivan; surely the very concept of a soccer mom and her requirements being different than a 9-5 commuter isn't lost on you?
    Reply
  • bigboxes - Monday, March 21, 2011 - link

    So, we can officially say this is the official tablet of soccer moms everywhere. Yay.

    She carries it around not because she is unlike Anand. She carries it around because she has a strong back!
    Reply
  • vol7ron - Sunday, March 20, 2011 - link

    A lot of the "sales" are from the retail outlets and not-necessarily the end-user consumer. There's people that buy it to sell to China or other Asian countries that buy it for double it's price; there are a plethora of reviewers these days; there are the people with mass amount of wealth that buy up anything just because they can; and then the hipsters that want to be cool and fit in. It reminds me of the episode of South Park with the smug Prius drivers.

    I'm not saying this isn't a bad device and it's mobility makes it beneficial in many regards. But the price of its mobility does not make it as attractive as it would be at the lower price (~$250). I'm not saying it should go for $100, but you're nearing the $1000 end of the spectrum for these devices and way over that for the necessary apps and accessories.
    Reply
  • crunc - Monday, March 21, 2011 - link

    I don't know why I'm getting into this argument, but all the iPads, including iPad 2's, that I'm seeing out in the world would seem to dispell your notion that no one is actually buying them for their own use. I saw 3 of them within 5 feet of me on the train this morning, for example. In 3 weeks time or so I'll be another one on the train with one, and also using it at home. I don't own a laptop. I wouldn't mind a laptop, but I'd rather have an iPad. It is, for me, far more comfortable to use then a laptop. Even the excellent trackpads on MacBooks don't compare to the entirely touch-based interface of the iPad. Obviously they aren't for everyone, but for some these are a great choice. I don't expect to write a book on it, but I then don't write books. If I ever decide to write a book, maybe I'll get a laptop. Reply
  • Ushio01 - Sunday, March 20, 2011 - link

    It's a fasion accessory just like the iphone, to be with the "in crowd" you have to have apple products that's all there is to it. Everyone on here must know at least someone who bought an iphone and then use it only for calls and texts, I know dozens of people who have done this. Reply
  • crunc - Monday, March 21, 2011 - link

    Actually, no, I don't know anyone who has an iPhone that only uses it for texts and phone calls. Everybody I know who has one uses it for virtually everything, myself included. In fact, I rarely text and only occasionally make phone calls (mostly of the, "should I pick up a pizza?" variety). You go on living in your little dream world, though. I won't stop you. I have an order in for an iPad 2 and I'm really looking forward to it. I love my iPhone and I want something akin to a laptop, but that isn't that, because the iOS interface is fantastic and the devices are more comfortable for me to use. Sure, there's some shortcomings to the platform, but they are overwhelmed by the multitude of positives. Reply
  • sarahtim - Sunday, March 20, 2011 - link

    I think this sort of comment represents a failure of imagination. As iPads sell million after million you have to adjust your idea of how many hipsters there are...
    Other people are different from you.
    Speaking for myself; I find my iPad extremely useful. I use it for a number of hours each day. I don't find it clunky. To me, and this is a very personal thing, the cost was of little consequence. While it is poor taste to blurt out your relative wealth when many folks are having a rough time of it, it is the only way to answer your comment. Further, I consider iPads to be very good value. I bought the bottom of the line iPad 1. It does everything I want. The bulk of its time is spent streaming video via the Air Video app.
    I represent a single data point - as do you. I fully appreciate that an iPad is a useless paperweight to you. No problem. When I use my iPad I do it in private. I don't discuss my ownership with others. I don't think I'm clever or a better person because I have one.
    You would have to look at me for a very long time before you thought of a hipster. Trust me on this. :-)
    Reply

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